Thenewstudyshowsthatsomeofthebluestragglershaveinsteadbeenrejuvenatedbyasortofcosmicfacelift, courtesyofcosmiccollisions. Thesestellarencountersarenearlyhead-oncollisionsinwhichthestarsmightactuallymerge, mixingtheirnuclearfuelandre-stokingthefiresofnuclearfusion. Mergedstarsandbinarysystemswouldbothbeabouttwicethetypicalmassofindividualstarsinthecluster.
Ourobservationsdemonstratethatbluestragglersformedbycollisionshaveslightlydifferentpropertiesfromthoseformedbyvampirism. Thisprovidesadirectdemonstrationthatthetwoformationscenariosarevalidandthattheyarebothoperatingsimultaneouslyinthiscluster, saysteammemberGiacomoBeccarifromESA.
Usingdatafromthenow-retiredWideFieldPlanetaryCamera 2 aboardHubble, astronomersfoundthatthesestragglingstarsaremuchmoreconcentratedtowardsthecentreoftheclusterthantheaveragestar. Thisindicatesthatbluestragglersaremoremassivethantheaveragestarinthiscluster, saysFerraro. Moremassivestarstendtosinkdeepintotheclusterthewayabilliardballwouldsinkinabucketofhoney.
Thecentralregionsofhighdensityglobularclustersarecrowdedneighbourhoodswhereinteractionsbetweenstarsarenearlyinevitable. Researchersconjecturethatoneortwobillionyearsago, Messier 30 underwentamajorcorecollapsethatstartedtothrowstarstowardsthecentreofthecluster, leadingtoarapidincreaseinthedensityofstars. Thiseventsignificantlyincreasedthenumberofcollisionsamongstars, andfavouredtheformationofoneofthefamiliesofbluestragglers. Ontheotherhand, theincreaseofstellarcrowdingduetothecollapseofthecorealsoperturbedthetwinsystems, encouragingthevampirismphenomenonandthusformingtheotherfamilyofbluestragglers. Almosttenpercentofgalacticglobularclustershaveexperiencedcorecollapse, butthisisthefirsttimethatweseetheeffectofthecorecollapseimprintedonastellarpopulation, saysBarbaraLanzoni, UniversityofBologna.
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